The present invention relates generally to the removal of surface layers of structural materials, especially concrete and masonry. More specifically, the present invention acts to remove surface layers of structural materials by placing controlled amounts of energetic materials (explosives) onto the surface and detonating the explosive in a controlled and safe manner. The present invention has special application to decontamination of chemical and nuclear facilities.
The removal of surface layers from a wall, ceiling, or floor by pulverization is known as scabbling. Scabbling is carried out for a number of purposes, including removing a worn or damaged surface layer for refinishing, to produce a rough surface on a structural member so as to insure bonding another member thereto, and removing contaminated surface layers from a structure so that the internal portions of the structure, which is uncontaminated, can be demolished or recycled using conventional techniques.
Of particular interest is the application of scabbling in decommissioning nuclear power plants and related facilities. The Department of Energy estimates that on the order of one trillion dollars will be required over the next 60 years in order to decommission obsolete nuclear facilities worldwide.
A major problem in such decommissioning is radioactive contamination of surfaces. Even though such contamination often does not penetrate into the central regions of structural materials, the entire mass of a building becomes mixed radioactive waste if conventional methods of demolition (e.g., explosive dismantling, wrecking ball, etc.) are used. The care required to handle such mixed waste is similar to that required to handle the contaminated surface material, if it were removed and handled separately. As the bulk of the building is much greater than that of the contaminated surfaces, perhaps an order of magnitude less contaminated waste need be generated, collected, transported, and disposed of if the contaminated surfaces can be easily removed.
If scabbling is to be used to remove contaminated surface layers from structural surfaces of a building, it is clear that the technique chosen must produce as small an amount of dust as possible in the process of rubbleizing the surface, and that the dust that is produced can be effectively collected and disposed of. Any dust produced during scabbling or later clean-up must be considered to be contaminated, and cannot be released to the environment. In particular, it cannot be allowed to spread through the building and recontaminate previously cleaned surfaces. In current practice, scabbling a concrete surface while maintaining acceptable dust management costs $50-250 per square meter of surfacexe2x80x94potentially tens of millions of dollars on a single facility. This does not consider costs associated with contamination of the machinery (usually large and complex) with which the scabbling is carried out.
Applications of scabbling are not restricted to decommission of nuclear facilities. Similar application can be made to contaminated chemical or biological facilities. In addition, scabbling can be used, e.g., on a worn concrete surface to remove the worn layer for subsequent replacement. Scabbling is often used with reference to concrete surfaces, but it can be applied to any structural material which is brittle and prone to fracture, including concrete, cement reinforced concrete, ceramics, tile, brick, cement block, stone, gypsum, and glass. For example, controlled scabbling will remove tile from a concrete subfloor. In milder forms, scabbling can even be used to remove corrosion from metal structural members.
Conventional scabbling techniques range over a wide range of technology. Simple mechanical insult is perhaps the standard, usually in the form of tungsten carbide penetrators driven by pneumatic pressure, although simple flails are sometimes used. Abrasive sprays and flails can also be used, although they are typically slower in action and produce enormous quantities of dust.
Mechanical shock waves can be used to spall flakes of the surface layers free from the body of the material. Among the methods which have been investigated for such removal are included pulsed laser irradiation, in which the sudden surface heating sends a shock wave into the material bulk, causing spallation of the surface. Another approach is electro-hydraulic excitation, in which an electric discharge sets up a shock wave in a fluid between the discharge electrodes. The shock wave is carried by the liquid to a nearby surface to be treated, where it causes spallation of the surface.
Shock wave based techniques for scabbling are quite promising, as they act to spall the material at natural weak spots, thereby reducing the surface layers to rubble of reasonably consistent dimensions with a minimum of dust formation. Unfortunately, the shock wave methods described above require complex, somewhat delicate, and rather expensive equipment to carry out the scabbling operation. In addition, with current technology only a very limited surface area can be treated using such techniques, owing to the huge energy densities (on the order of a kilojoule per cubic centimeter of scabbled material) required for effective scabbling.
There is thus a need for a new approach to scabbling surfaces of structural materials which preserves the beneficial features of the shock-wave techniques, but which can be carried out easily and inexpensively on large-scale projects. An additional need when scabbling is used for decontamination is to limit the amount of machinery required to carry out the scabbling operation, so as to reduce decontamination and replacement costs associated directly with the scabbling equipment.
The present invention addresses the above needs by using mild and relatively safe explosives to generate the shock waves needed to drive the scabbling process. Layers of explosives can be coupled to surfaces to be scabbled so that shock waves of the magnitude and duration required for efficient scabbling are produced therein. In addition, there is essentially no machinery exposed during the actual scabbling process (i.e., when the explosives detonate). Continual decontamination of complex machinery is thus avoided in the present invention.
Previous attempts to apply explosives for scabbling were applied in the form of flexible sheets of explosive (typically pentaerythritol tetranitrate (PETN) embedded in a rubber sheet). The results have been mixed, partially owing to the inevitable gaps between the explosive sheet and the surface to be scabbled. Intimate contact is required to effectively transmit the required shock wave into the (typically very rough) surface to be removed. Such contact could be approximated by using, e.g., prior art plastic explosives xe2x80x9cbutteredxe2x80x9d onto the surface. However, such formulations are difficult to apply with the delicacy required for controlled scabbling, and the extensive handling required dramatically raises both safety concerns and the ultimate cost of the scabbling operation.
The present invention solves the problem of contact with the surface to be scabbled by using explosive compositions which conform to the surface to be scabbled. In one implementation a thin layer of an explosive. composition is sprayed onto the surface. In another implementation, foamed explosives are applied to the surface. The compositions used are less energetic by volume than are conventional high explosives, and can be tailored to deliver a shock wave suitable for removing anywhere from a thin layer of concrete to a layer several inches thick. The rubble which results from use of the instant invention is conveniently sized for collection and disposal, and is generated with a minimum of dust.
The present invention pertains to a new approach to scabbling surfaces of structural materials. The essence of the new approach is to apply a thin layer of a mildly energetic explosive composition to the surface to be scabbled. The composition must be capable of sustaining a detonation front along the surface to which it is applied, but the intensity of the shock wave generated in the surface is typically less than that usually associated with conventional high explosives. The composition, when applied, conforms closely to the shape of the surface to be scabbled. For practicality, the composition should be stable under handling, easy to apply, easy to transport, have limited toxicity, and reliably detonate using conventional techniques.